An internal combustion engine, for example, a diesel engine, drives a camshaft and other engine accessories by the turning force of a crankshaft. Usually, the turning force of the crankshaft is transmitted to a fuel injection pump, a balance shaft, and a camshaft via an endless toothed belt.
A timing belt used in this belt drive device undergoes fluctuations in revolution from the camshaft or engine accessories during engine driving. Thus, a load on the timing belt increases or decreases.
For instance, a belt drive device of a diesel engine 100 is shown in FIG. 9. This belt drive device has a revolution transmission system for transmitting a turning force from a crank sprocket 101 to a cam sprocket 103 and a fuel injection pump sprocket 104 via a timing belt 102. The numeral 105 denotes an idler. If the diesel engine 100 is 4-cylindered, torque fluctuations of the crankshaft repeat a fluctuation pattern comprising two explosion torque fluctuations and two inertia torque fluctuations per revolution of the crankshaft as shown in FIG. 10. Torque fluctuations of the camshaft, on the other hand, repeat a specific fluctuation pattern for each cylinder as shown in FIG. 11. Furthermore, torque fluctuations of the fuel injection pump repeat a specific fluctuation pattern per injection as shown in FIG. 12.
Thus, as shown in FIG. 13, revolution fluctuations of the crankshaft follow a line a, revolution fluctuations of the camshaft follow a line b, and revolution fluctuations of the fuel injection pump follow a line c, in the entire revolution speed range of the diesel engine 100. In this case, the load on the timing belt 102 changes as shown in a solid line A on the tension side of the fuel injection pump sprocket 104. Particularly at a predetermined engine speed Ne.sub.1, resonance occurs, maximizing the load on the timing belt.
That is, a string such as the timing belt (the tension side of the fuel injection pump sprocket) is known to resonate. At the time of resonance, the load is known to peak. It is also known that its basic resonance frequency is inversely proportional to the length of the string, and is proportional to the square root of the ratio of the tension of the string to the linear density of the string.
With the belt drive device, the timing belt load is thus maximal at resonance. In setting the strength of the timing belt, and for ensuring its durability, therefore, the sectional shape, tension, material, and string vibration interval of the timing belt are suitably set so that the timing belt will have a sufficiently larger allowable limit of load P.sub.max than the timing belt load Ph. Besides, the length, tension, and linear density of the string are selected so that the engine speed Ne.sub.1 at resonance is excluded from the normal engine speed range. Actually, however, the belt drive device is required to have full durability, and yet is subject to restrictions on layout. The present situation is that sufficiently effective measures cannot be taken.
In reducing the timing belt load of such a belt drive device, it is known to use a dynamic damper. FIG. 14 shows that an inertial body B with the moment of inertia I.sub.B is supported on the substrate side via an elastic body with a spring constant K.sub.B, and an inertial body D with the moment of inertia I.sub.D is connected to the inertial body B via an elastic body with a spring constant K.sub.D. In this case, the inertial body D vibrating at a torsion angle .theta..sub.D acts as a dynamic damper d on a vibration system b in which the inertial body B vibrates at a torsion angle .theta..sub.B.
Basically, if the natural frequency of the dynamic damper d is set at a value equal to the frequency of a vibrating force (torsional vibration) received by the inertial body B, only the dynamic damper d can be caused to vibrate. Actually, the vibration system b vibrating at the torsion angle .theta..sub.B, as shown in FIG. 14, is known to produce two resonance peaks p' (see a dashed line in FIG. 15) which are at a lower level than a resonance peak p obtained in the absence of the dynamic damper d. The natural frequency fd of the dynamic damper d at this time is given by the equation (1) below. If the dynamic damper d is prepared such that this natural frequency fd agrees with the resonance frequency of the torsional vibration system b, the torsional vibration of the torsional vibration system b is damped. EQU fd=(1/2.pi.).times..sqroot.(K.sub.D /I.sub.D) (1)
A device using such a dynamic damper to absorb the resonance of a timing belt is disclosed, for example, in Japanese Laid-Open Patent Publication No. 20950/87.
According to the device disclosed in this publication, a dynamic damper is mounted on a camshaft sprocket in order to prevent the situation that a timing belt resonates to cause revolution fluctuations to the camshaft, disordering the ignition timing of an ignition device linked to the camshaft, and eventually lowering engine output.
In this device, when the timing belt vibrates like a string, a tensile force (timing belt load) works on the belt twice per period. In this view, during the resonance of the timing belt, vibrations of a frequency twice the resonance frequency are exerted on the camshaft sprocket side. Thus, the dynamic damper used in the device is one vibrating in a direction opposite to the vibrations of the timing belt and at a natural frequency twice the resonance frequency of the timing belt. Consequently, vibrations of the timing belt are counteracted by vibrations given by the dynamic damper, thereby preventing the disorder of the ignition timing of the ignition device linked to the camshaft.
Incidentally, if, in addition to the camshaft, other engine accessory, such as a fuel injection pump with a high drive torque, is disposed in the revolution transmission system of the belt drive device, the following problem arises:
To achieve improvements in exhaust gas from a diesel engine, there is a growing demand for a finer fuel spray, and an increase in the fuel injection pressure is desired. Thus, the drive torque of the fuel injection pump tends to increase further. As a result, load on the timing belt increases, and the range of torque fluctuations of the fuel injection pump, in particular, widens. This produces a tendency toward a greater load on the timing belt during its resonance.
To ensure the strength of the timing belt, an attempt is made to broaden its sectional shape. However, since there is a restriction to the layout of the engine room, increases in the belt width and thickness pose many problems, and the improvement of the material is also limited. To exclude the resonance frequency from the normal revolution speed range, it is also attempted to use, say, an idler pulley in setting the string vibration interval of the timing belt at a relatively small value. However, exclusion of the resonance frequency from the normal revolution speed range is actually often difficult.
Under these circumstances, a belt drive device capable of dealing with the increase in the belt load on the timing belt is desired.
In a belt drive device as illustrated in FIG. 9, it may be tried to deal with the increase in the load on the timing belt without decreasing the durability of the timing belt. In this case, it is necessary to increase the belt width, etc., but a restriction is imposed on the layout, as stated above.
The belt drive device disclosed in Japanese Laid-Open Patent Publication No. 20950/87 also suppresses belt vibrations by the dynamic damper to reduce torque fluctuations which the camshaft undergoes. Truly, torque fluctuations of the camshaft can be decreased. However, if an engine accessory requiring a greater driving force, such as a fuel injection pump, is disposed in the revolution transmission system of the timing, the belt load on the timing belt on the tension side of the fuel injection pump increases. The belt load at resonance, in particular, sharply increases, posing the problem that full durability of the timing belt cannot be secured.
It is an object of the present invention to provide a belt drive device capable of ensuring the sufficient durability of a timing belt even when a plurality of engine accessories are mounted in the revolution transmission system of the timing belt.